The lead-acid storage battery is commonly found in two modes of design: the valve-regulated recombinant cell and the flooded cell. Both modes include positive and negative electrodes typically in the form of plates that are separated from each other by a porous battery separator. The porous separator prevents the electrodes from coming into physical contact and provides space for an electrolyte to reside. Such separators are formed of materials that are chemically resistant to the sulfuric acid electrolyte and are sufficiently porous to permit the electrolyte to reside in the pores of the separator material, thereby permitting ionic current flow with low resistance between adjacent positive and negative electrode plates.
More recently, enhanced flooded batteries (EFB) have been developed to meet the high cycling requirements in “start-stop” or “micro-hybrid” vehicle applications. In such applications, the engine is shut off while the car is stopped (e.g., at a traffic light) and then re-started afterwards. The advantage of a “start-stop” vehicle design is that it results in reduced CO2 emissions and better overall fuel efficiency. A major challenge to the operation of “start-stop” vehicles is that the battery must continue supply of all electrical functions during the stopped phase while supplying sufficient current to re-start the engine at the required moment. In such cases, the battery must exhibit higher performance with respect to cycling and recharge capability as compared to that of a traditional flooded lead-acid battery. In the case of “start-stop” applications, valve-regulated lead-acid (VRLA) batteries have demonstrated good cycleability in the field, but they suffer from relatively high cost and lower performance in high temperature environments. As such, EFB batteries offer reduced cost and an opportunity to overcome the limitations of VRLA batteries.
Separators for lead-acid storage batteries have been formed of different materials as the technology has developed. Sheets of wood, paper, rubber, PVC, fiberglass, and silica-filled polyethylene have all found use over time. Currently, absorptive glass mat (AGM) separators are used in VRLA batteries, while silica-filled polyethylene separators are used in automotive starting-lighting-ignition (SLI) batteries. In the latter separators, the microporous polyethylene separator web contains a large fraction of silica particles that function as a wettability component to provide wettability for the acid electrolyte and to help define the pore structure of the separator. A separator of this type is described in U.S. Pat. No. 7,211,322.
In the case of EFB batteries, it is desirable to use a silica-filled polyethylene separator that has exceptionally low electrical resistivity, maintains wettability, mitigates acid stratification, and exhibits good mechanical properties and high oxidation resistance. Conventional silica-filled polyethylene separators used in SLI batteries do not meet all the requirements for EFB batteries.
As such, there continues to be a need for a low-cost, ultralow resistance, silica-filled polyethylene separator that exhibits permanent wettability throughout the cycle life of lead-acid storage batteries used in EFB applications.